Initially, the first shoulder prostheses, so-called NEERs, consisted of monobloc prostheses, i.e., they comprised a prosthesis stem and a head in a single piece. However, experience has shown that such prostheses have two major disadvantages.
As a matter of fact, and in the first place, the one-piece nature of these prostheses requires a large inventory of different implants that can adapt to the anatomical requirements of various patients. Furthermore, since the relative position of the head and the prosthesis stem are fixed, and also because of the one-piece nature of the prosthesis, the only solution available to the practitioner in order to accurately reproduce the anatomy of the upper end of the humerus consists of fitting the humerus to the prosthesis, an operation that proves to be quite tricky and a source of failures, or at least future traumatisms, for the patient.
Several solutions have been proposed to mitigate these disadvantages, as described in particular in French patents FR-2 685 633, FR-2 727 002, FR-2 727 857, and FR-2 731 612, aimed at providing prostheses with heads and prosthesis stems linked through connecting devices whose relative position can be adjusted by rotating the head around two or three axes of rotation with respect to the prosthesis stem, then locking these elements in the desired configuration.
Compared to the NEER prostheses, these prostheses have the advantage of making it possible to approximate the exact anatomy of the upper end of the humerus. However, they also have two major disadvantages.
As a matter of fact, and in the first place, due to the design of the systems for the related locking of the head and the prosthesis stem, they first require that the practitioner uses trial prostheses in order to determine the ideal relative positioning of the head and the prosthesis stem, and then duplicates the geometry of the trial prosthesis in order to construct the final prosthesis that is to be implanted.
But this surgical technique leads to an increase in the length of the operation during which the patient is under anesthesia. Furthermore, it can be the source of errors prejudicial to the patient during the duplication of the trial prosthesis, which must be done during the operation, i.e., as quickly as possible. It should also be noted that due to the obligation of duplicating the trial prosthesis, any increased possibility of adjustment that would advantageously make it possible to provide a better reproduction of the anatomy of the upper end of the humerus leads to complicating the step of duplicating this test prosthesis. Thus, it appears in particular that this duplication step is most difficult for the prostheses described in patents FR-2 727 002 and FR-2 727 857, which offer the best-performing range of adjustments.
The second disadvantage of these prostheses results from the fact that anatomical studies have made it possible to reveal the existence of a double eccentricity, called combined offset, between the proximal metaphysical axis of the humerus and the parallel axis that passes through the center of the humeral head. This combined offset has a medial component, called medial offset, that is relatively constant from one individual to another, and a posterior component, called posterior offset, that is relatively variable from one individual to another, between which there is no relationship. And none of the current prostheses as described in particular in the aforesaid French patents makes it possible to adjust the medial offset and the posterior offset independently, so that an exact reproduction of the upper end of the humerus cannot be achieved to this day. Furthermore, this impossibility of independent adjustment of the two offsets may lead the practitioner to artificially increase the humeral retro-torsion during implantation.